1. Field of the Invention
The present invention relates to a method of operating an acid digestion kettle of tantalum or tantalum-base alloy for digestion of organic waste matter and more particularly relates to a method of ensuring stabilized continuous operation of an acid digestion kettle of tantalum or tantalum-base alloy by preventing advance of deterioration and embrittlement thereof due to hydrogen absorption.
2. Description of the Prior Art
In facilities handling nuclear fuel or other radioactive isotopes in general, it is necessary to protect the human body from exposure. Rubber products made by polymerization or copolymerization of chlorine-containing monomers such as vinyl chloride, vinylidene chloride and chloroprene are typical organic raw materials having radiation shielding ability. Body protectors made of these raw materials have to be treated for disposal after being used for long periods of time because of contamination by radioactive rays. Methods of treating such organic products contaminated by radioactive rays include the method of subjecting them to volume decrease and stabilization and then to compaction for convenient storage. There are several methods for performing this volume decrease and stabilization method, and the most effective one under investigation is to put the said organic products into a high temperature high concentration sulfuric acid bath to subject them to acid digestion. This method includes the following two treating steps, the conditions therefor being very severe.
(1) Decomposition of organic materials by sulfuric acid. PA0 (2) Oxidation of carbon by addition of nitric acid. EQU 3C+4HNO.sub.3 .fwdarw.4NO+3CO.sub.2 +2H.sub.2 O
(In the case of polyvinyl chloride) EQU (C.sub.2 H.sub.3 Cl).sub.n +nH.sub.2 SO.sub.4 .fwdarw.2nC+nSO.sub.2 +2nH.sub.2 O+nHCl
The first treating step is usually performed in an 85% sulfuric acid bath at 200.degree. C. or above for about 5 hours, followed by addition of nitric acid for the second treating step, which requires about 2 hours.
As for materials which can endure such severe conditions, platinum, high silicon cast iron, glass lined steel and tantalum (including tantalum-base alloys, hereinafter referred to briefly as Ta) are included. However, platinum is expensive, high silicon cast iron is brittle, and glass lined steel is difficult to process into a complicated shape and is liable to crack when subjected to thermal history. Thus, the remaining material, Ta must be considered. However, Ta is liable to corrode, though very little, and embrittle when it absorbs hydrogen resulting from such corrosion. Unless these problems are solved, therefore, Ta cannot be used as a material for acid digestion kettles.
Heretofore, little research on prevention of Ta corrosion has been conducted and this field of research remains substantially untouched. A known method of prevention of hydrogen absorption embrittlement is to stick a platinum foil to a portion of Ta surface to produce a kind of sacrificial cathode effect. However, corrosion weight loss of Ta advances so rapidly as to require frequent replacement of the foil, which is disadvantageous from the standpoints of efficiency of operation and economy.